The present invention relates to methods for communications between terminals in a centralized supervisory system, and more particularly to a method for a communication between terminals in a centralized supervisory system equipped with a centralized supervisory device which collects supervisory information included in control signal cells sent from individual supervisory devices and sends main signal cells to a network.
Recently, communications in an asynchronous transfer mode (ATM) have been in the spotlight as a communication method due to diversification of communication services. When an ATM communication takes place, an ATM terminal assembles a cell of a setup message including a sender number and a receiver number and sends the cell to an ATM network. Upon receiving the cell for setup, the ATM network establishes a VCC (Virtual Channel Connection) between the sender terminal and the receiver terminal, and notifies the sender and receiver terminals of respective connection identifiers VPI and VCI (Virtual Path Identifier and Virtual Channel Identifier) used in the communication. Then, the sender terminal segments data into cells and adds the notified connection identifiers to the header of each cell. Then, the cells are sent via the line. Each switch in the ATM network performs switching while changing the values of the connection identifiers of the input cells. The receiver terminal accepts the cells each having the specified connection identifiers, and disassembles the cells into data.
A plurality of ATM terminals can be accommodated in the ATM network as follows. The ATM terminals are connected to first transmission devices which implement an UNI (User Network Interface). The first transmission devices are connected to a second transmission device, which is connected to the ATM network. In such a structure, an individual supervisory device provided in each of the first transmission devices supervises the line quality (performance) and a fault state (alarm), and sends the supervisory information thereon to a centralized supervisory device provided in the second transmission device. The centralized supervisory device collects and manages the supervisory information sent from the supervisory devices, and outputs it to an external device such as a personal computer.
FIG. 15 is a block diagram of a conventional centralized supervisory system in which the supervisory function of the system is mainly illustrated. Referring to FIG. 15, there are illustrated individual supervisory devices (first transmission devices) 1.sub.1 -1.sub.n, a centralized supervisory device (second transmission device) 2, an ATM network 3, and an MMI (Man Machine Interface) unit 4. A large number of ATM terminals are connected to each of the individual supervisory devices 1.sub.1 -1.sub.n, which are connected to the centralized supervisory device. The MMI unit 4 is a personal computer, a workstation or the like.
FIG. 16 schematically shows a communication part of each of the individual supervisory devices 1.sub.1 -1.sub.n. The communication part is made up of a supervisory information memory 1a, a processor (CPU) 1b, a frame assemble/cell separate part 1c, and a cell assemble/disassemble part 1d. The supervisory information memory 1a stores the collected supervisory information. The supervisory items of the individual supervisory devices include the fault state (alarm) and the line quality (performance). The frame assemble/cell separate part lc assembles the cells (the main signal cells and control signal cells) into a frame which conforms with, for example, the SONET OC3 level, and sends the OC3 frame to a transmission line (optical fiber) TL. Further, the frame assemble/cell separate part 1c separates the received OC3 frame into the main signal cells and control signal cells. The cell assemble/disassemble part 1d assembles a control signal from the CPU 1b into control signal cells, and disassembles control signal cells supplied from the frame assemble/cell separate part 1c into the control signal, which is input to the CPU 1b. The control signal cells are cells which are used to transfer the supervisory information between the individual supervisory devices 1.sub.1 -1.sub.n and the centralized supervisory device 2, and is equipped with a particular connection identifier (VPI/VCI) directed to discriminating the control signal cell from the main signal cells. The main signal cells are cells which are received from the ATM terminals and are sent thereto.
FIG. 17 is a diagram of a frame format of the SONET OC3 signal. One frame consists of 9.times.270 bytes. The first 9.times.9 bytes form the section overhead SOH, and the remaining bytes form the path overhead POH and the payload PL. The section overhead SOH is used to transmit information indicating the beginning of the frame (framing signal), information inherent in the transmission line (information concerning error on the transmission line and information for maintenance of the network), and a pointer indicating the position of the path overhead POH. The path overhead POH is used to transmit end-to-end supervisory information in the network. The payload PL is used to transmit information at a bit rate of 150 Mbps so that a large number of cells (main signal cells and control signal cells) CL1-CLn are mapped therein.
FIG. 18 schematically shows a communication part of the centralized supervisory device 2. The communication part shown in FIG. 18 is made up of a supervisory information memory 2a, a processor (CPU) 2b, a frame assemble/cell separate part 2c, and a cell assemble/disassemble part 2d. The supervisory information memory 2a stores the supervisory information collected from the individual supervisory devices 1.sub.1 -1.sub.n. The frame assemble/cell separate part 2c assembles the cells (the main signal cells and control signal cells) into a frame which conforms with, for example, the SONET OC3 level, and sends the OC3 frame to a transmission line (optical fiber) TL. Further, the frame assemble/cell separate part 2c separates the OC3 frame received over the transmission line TL into the main and control signal cells. The cell assemble/disassemble part 2d assembles a control signal from the CPU 2b into control signal cells, and disassembles control signal cells supplied from the frame assemble/cell separate part 2c into a control signal, which is then input to the CPU 2b.
FIG. 19 is a diagram of a supervisory sequence in which the centralized supervisory device 2 collects the supervisory information from the individual supervisory devices 1.sub.1 -1.sub.n. The CPU 2b of the centralized supervisory device 2 sends a polling cell to the individual supervisory device 1.sub.1 via the cell assemble/disassemble part 2d and the frame assemble/cell separate part 2c. The frame assemble/cell separate part 1c of the individual supervisory device 1.sub.1 supplies the received polling cell to the CPU 1b via the cell assemble/disassemble part 1d. The CPU 1b checks whether there is a change in the supervisory information between the previous polling and the current polling. If it is judged that there is no change, the CPU 1b sends an ACK cell to the centralized supervisory device 2 via the cell assemble/disassemble part 1d and the frame assemble/cell separate part 1c.
The CPU 2b of the centralized supervisory device 2 receives the ACK signal via the frame assemble/cell separate part 2c and the cell assemble/disassemble part 2d, and thus recognizes that there is no change in the supervisory information on the individual supervisory device 1.sub.1. Then, the CPU 2b sends the polling cell to the next individual supervisory device 1.sub.2. If there is a change in the supervisory information on the individual supervisory device 1.sub.2, the CPU 1b thereof assembles changed supervisory information (change data) into the cell, which is then sent to the centralized supervisory device 2. Then, the centralized supervisory device 2 extracts the change data from the received cell, and revises the old data in the supervisory information memory 2a by using the change data. The frame assemble/cell separate part 1c of the individual supervisory device 1.sub.2 inserts the control signal cells (supervisory information cells) into the main signal cells at a fixed rate. Then, the main signal cells are framed and sent to the transmission line. That is, the frame assemble/cell separate part 1c maps a number of control signal cells corresponding to the fixed rate in the payload PL of the SONET OC3, and sends the mapped control signal cells to the transmission line. The CPU 2b of the centralized supervisory device 2 receives all change data, and thereafter sends the polling cell to the next individual supervisory device 1.sub.3. Then, the supervisory information is collected and managed in the same sequence as described above.
The conventional communication method has an advantage in terms of cost because control paths (VPI/VCI) are provided in the main signal cables connecting the first transmission devices (individual supervisory devices) 1.sub.1 -1.sub.n and the second transmission device (centralized supervisory device) located in remote areas and there is thus no need for cables specifically used for the control paths.
However, the conventional communication method handles a large number of items of the supervisory information. Hence, the conventional communication method in which the control signal cells are sent at the fixed rate has a disadvantage in that, if an individual supervisory device starts to send a large amount of data, the polling signal will be sent to the next supervisory device with a time delay.
There is another disadvantage in that the centralized supervisory device 2 sends, with a time delay, a command required for real-time performance to the individual supervisory device which starts to send a large amount of data.